Radiography is utilized for medical diagnosis, industrial non-destructive examination and the like. An intensifying screen/film method is generally applied to radiography. The intensifying screen/film method is a method for obtaining an X-ray image by converting x-rays transmitted through a specimen into visible light by an intensifying screen, and photosensitizing a film by this visible light. Medical diagnosis, non-destructive examination and the like are carried out based on the X-ray images formed on the films.
Meanwhile, a method for performing examination by converting an X-ray image into an electric signal and performing image processing for this electric signal is becoming pervasive as a result of the advance of the digital technology in recent years. Medical diagnosis, non-destructive examination and the like are carried out based on the visible image (X-ray image) displayed on a CRT or the like. As a method for performing inspection by digitalizing an X-ray image, a method of using an X-ray image intensifier (I. I. tube) as an X-ray detector is known.
The X-ray image intensifier is provided with an input part for converting X-rays into photoelectron and an output part for converting photoelectron into visible light at both end parts of a vacuum envelop. The visible light image which is outputted is digitalized by a CCD camera or the like. For example, in the diagnosis of a lung, the image of an area of about 400×400 mm is picked up. In order to ultimately detect the examination image of such a image taken area by a CCD camera of about one inch, it is necessary to highly concentrate light in an I. I. tube. Therefore, an I. I. tube causes the problem of increasing the size of apparatus.
With the advance of the semiconductor process technology in recent years, a system for performing radiography by using semiconductor sensors and semiconductor elements arranged in an array form is developed. This kind of radiography system can easily cope with a wide-ranged image pickup area and can directly digitalize the image data. Therefore, this is suitable for putting radiographic data (image data) into database and the like in the medical field, for example. Further, it has the advantage of having a larger dynamic range as compared with the X-ray photograph system using the conventional photosensitive film so that the image accuracy is not influenced by the variation in the X-ray exposure amount.
In the radiography system utilizing the semiconductor process technology, an X-ray plane detector in which an amorphous silicon thin-film transistor (a-Si TFT) in an array form is applied to a switching gate is used. As the X-ray plane detectors, a direct conversion type using an X-ray charge conversion film for directly converting an X-ray image into electric charge information and an indirect conversion type using a phosphor layer for converting the X-ray image into an optical signal and a photoelectric conversion film for converting an optical signal into electric charge information are known (see Japanese Patent Application Laid-open No. 2002-090460, Japanese Patent Application Laid-open No. 2002-286846 and the like).
In a direct conversion type X-ray plane detector, substances which have adverse effects on a human body and environment, such as a-Se, PbI2, and HgI2 are used for the X-ray charge conversion film, and therefore, an indirect conversion type X-ray plane detector is being developed for commercial use. The indirect conversion type X-ray plane detector has the structure in which a fluorescence emitting part emitting light by X-rays is disposed on an optical detection part which is the combination of, for example, a photoelectric conversion film and a TFT as a switching gate. As the fluorescence emitting part, a phosphor sheet with a phosphor coated layer formed on, for example, a sheet-shaped base support is used.
A praseodymium-activated rare earth oxysulfide phosphor such as Gd2O2S: Pr, a terbium-activated rare earth oxysulfide phosphor such as Gd2O2S: Tb and the like are used for fluorescence emitting parts of such X-ray plane detectors as shown in Japanese Patent Laid-open Application No. 2002-090460 and Japanese Patent laid-open Application No. 2002-286846. They are the phosphors which are generally used for X-ray detectors due to high emission efficiency by X-rays.
However, the phosphor sheets using the conventional Gd2O2S: Pr phosphor, the Gd2O2S: Tb phosphor and the like have the problem of being incapable of sufficiently enhancing sensitivity of the X-ray detectors including photoelectric conversion films. From the above reason, in the conventional indirect conversion type X-ray plane detector, only the sensitivity equivalent to or less than the intensifier screen/film type and the image pickup system using an X-ray image intensifier (I. I. tube) can be obtained under the present circumstances. The conventional indirect conversion type X-ray plane detector can obtain only the characteristic equivalent to or less than the conventional image pickup system concerning definition of an image.
An object of the present invention is to provide a phosphor sheet for a radiation detector enhanced in the characteristic when used in combination with a photoelectric conversion film. More specifically, an object of the present invention is to provide a phosphor sheet for a radiation detector which makes it possible to enhance detection sensitivity of radiation, definition and the like of a radiation image. Another object is to provide a radiation detector and an apparatus for radiographic examination which intend to enhance accuracy, definition and the like of a radiation image by using such a phosphor sheet.